Throughout the 1900xe2x80x3s, use of the sun as a source of energy has evolved considerably. Early in the century, the sun was the primary source of interior light for buildings during the day. Eventually, however, the cost, convenience, and performance of electric lamps improved and the sun was displaced as our primary method of lighting building interiors. This, in turn, revolutionized the way we design buildings, particularly commercial buildings, making them minimally dependent on natural daylight. As a result, lighting now represents the single largest consumer of electricity in commercial buildings.
During and after the oil embargo of the 1970s, renewed interest in using solar energy emerged with advancements in daylighting systems, hot water heaters, photovoltaics, etc. Today, daylighting approaches are designed to overcome earlier shortcomings related to glare, spatial and temporal variability, difficulty of spatial control and excessive illuminance. In doing so, however, they waste a significant portion of the visible light that is available by shading, attenuating, and or diffusing the dominant portion of daylight, i.e., direct sunlight which represents over 80% of the light reaching the earth on a typical day. Further, they do not use the remaining half of energy resident in the solar spectrum (mainly infrared radiation between 0.7 and 1.8 um), add to building heat gain, require significant architectural modifications, and are not easily reconfigured. Previous attempts to use sunlight directly for interior lighting via fresnel lenes collectors, reflective light-pipes, and fiber-optic bundles have been plagued by significant losses in the collection and distribution system, ineffective use of nonvisible solar radiation, and a lack of integration with collocated electric lighting systems required to supplement solar lighting on cloudy days and at night.
Similar deficiencies exist in photovoltaics, solar thermal electric systems, and solar hot water heaters. FIG. 2 shows the conversion efficiency of traditional silicon-based solar cells in the ultraviolet and short wavelength visible region of the solar spectrum is low and the solar energy residing beyond xcx9c1.1 um is essentially wasted. To overcome this and address other economic barriers, one approach has been to develop utility-scale photovoltaic (PV) and solar thermal concentrators. The rationale being that the cell area and, consequently, the cell cost can be reduced by approximately the same amount as the desired concentration ratio. Unfortunately, this cost-savings is typically offset by the added cost and complexity of the required solar concentrator and tracking system.
In recent years, researchers have begun developing photobioreactors that use sunlight-induced photosynthesis to sequester carbon to produce biofuels such as hydrogen using cyanobacteria. Large-scale photobioreactors are already indispensable in the successful commercial production of phototrophic unicellular algae valued in such markets as aquaculture, pharmaceuticals, animal-feed additives and health foods. Unfortunately, very little of the incident sunlight is tapped to maximize cyanobacteria growth rates, and only 10% of the energy residing in the visible portion of the spectrum is typically used productively to produce biomass. Terrestrial solar radiation can reach xcx9c2000 xcexcE m2 sxe2x88x921, which easily satisfies the photosynthetic photon flux (PPF) requirements of algae. Indeed, at elevated PPF levels (greater than xcx9c200 xcexcE m2 sxe2x88x921), the kinetic imbalance between the rate of photon excitation and thermally-activated electron transport results in saturation of the photosynthetic rate. In the case of thermophilic and mesophilic cyanobacteria that are ideally-suited for carbon sequestration because of their thermal adaptation to higher temperatures, even a lower PPF level (xcx9c6 100 uEm2sxe2x88x921) is required to achieve maximum carbon fixation. Thus, most of the lighting energy available from solar irradiance goes unused.
The principal hurdle to the scale up of photobioreactors to achieve a viable commercial-scale production of algae is lighting limitation, both in terms of light delivery and distribution and energy expenditure. For instance, current methods for mass cultivation of marine microalgae include translucent fiberglass cylinders, polyethylene bags, carboys and tanks under artificial lighting or natural illumination in greenhouses. In these cases, however, at an algal density of 0.45 g/L, for example, light penetrates the suspension only to a depth of 5 cm, leaving a significant percentage of the cells in complete darkness at any given time. As such, microalgal production in these systems seldom exceeds 100 kg DW per year per facility, and maintaining these systems is labor- and space-intensive and quite unreliable. Moreover, when lighting is provided by artificial lamps (such as fluorescent, high-pressure sodium or incandescent) in close proximity to the bioreactor vessel, the comparatively poor luminous efficacy and dissipation of heat from the lamps present a constant problem.
Natural bioreactors using traditional raceway cultivators commonly waste 90 to 95% of the incident photosynthetic photon flux at high algal densities along with the remaining solar energy resident in the UV and IR portion of the spectrum. This equates to an overall solar energy utilization factor of 2.5 to 5%, making conventional photobioreactors very difficult to justify from a cost and performance perspective.
The approach first demonstrated in Japan to improve the sunlight utilization efficiency of natural photobioreactors is to collect, transport, and distribute sunlight over a larger surface area, thereby improving the sunlight utilization efficiency by reducing losses caused by saturation. The concept included the use of the earlier-mentioned fresnel-lens sunlight collector and a fiber optic bundle system to transport and distribute the light. Losses in the visible-light collection, transport and distribution system were typically more than 75%, and the 2xc3x97-to-3xc3x97 improvements in sunlight utilization was far outweighed by the added cost ($5,000/m2 of sunlight collected). This approach serves as partial precursor to this invention.
This invention improves the total end-use power displacement efficiency of solar energy by integrating solar technologies into multi-use hybrid systems that better utilize the entire solar energy spectrum. As illustrated in FIG. 3, a primary mirror concentrates the entire solar spectrum onto a secondary optical element where the visible portion of the solar spectrum is separated from the UV and near infrared portions. The two energy streams are used for different purposes, i.e. lighting and electricity generation or process heat.
This adaptive, full-spectrum (AFS) solar energy system is a unique alternative to solar energy use in buildings and photosynthetic-based bioreactors. It uses solar energy from a dynamic, systems-level perspective, integrates multiple interdependent technologies, and makes better use of the entire solar energy spectrum on a real-time basis.
The solar system uses a hybrid solar concentrator, shown in FIG. 3, that efficiently collects, separates, and distributes the visible portion of sunlight while simultaneously generating electricity from the infrared portion of the spectrum using new gallium antimonide (GaSb) infrared thermophotovoltaics (IR-TPVs). The optical and mechanical properties of improved large-core polymer optical fibers more efficiently deliver large quantities of visible sunlight into buildings and photobioreactors. Once delivered, the visible sunlight is used much more effectively than previously to illuminate building interiors using new hybrid luminaires. Improved cyanobacteria growth rates, packing densities, and solar utilization efficiencies in hybrid solar photobioreactors that use fibers to more efficiently distribute and use light that would have otherwise been wasted via photosynthetic saturation is also provided.
This invention redirects and more-efficiently uses portions of the solar energy spectrum originating from a common two-axis, tracking solar concentrator in real-time using electro-optic and or opto-mechanical devices. Analytical/experimental models and intelligent control strategies enhance the use of adaptive full-spectrum solar energy systems in its two primary applications i.e. commercial buildings (also illustrated in FIG. 1) and hybrid solar photobioreactors used to mitigate CO2 at power plants (also illustrated in FIG. 1).
This invention uses: a) advanced materials including GaSb thermophotovoltaics, and spectrally-selective UV cold mirror thin film coatings, b) biomass resource development through innovative approaches to improve sunlight utilization in photobioreactors used in carbon sequestration and the production of fuels, chemicals, and agriculture products, c) intelligentsensor/control systems for use in adaptive solar energy systems in commercial buildings, d) computational science tools to aid in the design of adaptive full-spectrum solar energy systems, model application-specific dynamic system behavior, and predict/optimize performance, and e) distributed power conversion systems for use in buildings and new hybrid solar photobioreactors.